Study On Low Temperature Reforming Of Liquefied Petroleum Gas Over Mg-Al Mixed Oxide Supported Nickel Catalysts

Polymer electrolyte membrane fuel cells （PEMFC） have attracted considerableinterest for transport and stationary applications due to their low operatingtemperature, high efficiency, low or null greenhouse gas emissions, fast startup andshutdown. Steam reforming of natural gas is currently the most economical processfor producing hydrogen. As one of the options, liquefied petroleum gas （LPG） arepotential sources for low cost hydrogen production for fuel cell applications becauseof their higher energy density than natural gas, easy storage and well-establisheddistribution infrastructures. In this paper, a series of yNi/MgmAl catalysts withdifferent Mg/Al （m=0.5,1.25,2,3） molar ratios and Ni loading （y=5%,10%,15%,20%） were prepared by co-precipitation-impregnating method and used for lowertemperature steam reforming of LPG. The Ni/MgmAl catalysts were characterized byBET, TEM, TPR, and TG. The effects of Mg/Al molar ratios, Ni loadings, supportsand catalysts calcination temperatures on the catalytic behavior were investigated indetail.（1） The precursors of the MgmAl supports showed a typical layered doublehydroxide （LDH） structure and were transformed to cubic MgO-like phase aftercalcined at500oC. The crystalline particle sizes and crystallinity increased withincreasing Mg/Al moral ratio and calcination temperature. For the Ni/MgmAl catalysts,Ni species were highly dispersed on the catalyst surfaces and no NiAl₂O₄spinal wasformed. The yNi/Mg_mAl catalysts could be reduced to metallic Ni nanoparticles at650oC.（2） The calcination temperature of the MgmAl support had certain influence onthe structure and catalytic activity to the catalysts for LPG pre-reforming. The Nispecies had strong interaction with the supports at the lower calciantion temperature,resulting in more difficult reduction of Ni species. However, higher calcinationtemperature of the support caused the formation of larger Ni particles due to a weak interaction. The results revealed that the Ni/MgmAl catalyst with the support calcinedat500oC had the optimal catalytic performance for LPG pre-reforming.（3） The effect of the calcination temperature of the catalyst on the structure andactivity for LPG pre-reforming was also investigated. At lower calcinationtemperature, there were free NiO species on the catalyst surface, which could causelarger Ni particles after reduction, producing lower catalytic activity and stability.With the increase in the temperature, the interaction between NiO and the support wasincreased. When the temperature was elevated to700oC, NiO speices were difficult tobe reduced into metalic Ni. It was proper to use the Ni/MgmAl catalyst calinced at500oC for LPG pre-reforming.（4） The catalytic behavior of the Ni/MgmAl catalysts with different Mg/Al andNi content for steam reforming of LPG was investigated. A proper increase in Mg/Almolar ratio could increase the basicity on the catalyst surface, resulting in theimprovement of the resistance to coke and stability of the catalysts. However, higherMg/Al molar ratio enhanced the interaction between the supports and Ni species,inhibiting the reduction of Ni2+and decreasing the catalytic activity. LPG conversionincreased with the increase in Ni loading. However, when Ni loading was more than15%, the catalytic activity started to decline. The results revealed that the15%Ni/Mg_1.25Al catalyst had the optimal catalytic performance for LPGpre-reforming.（5） The influences of the reaction conditions on the catalytic performance weresystematically studied. High S/C molar ratio favored conversion of LPG to small gasmolecules and enhanced the resistance to coke deposition. Methane content in the exitgas decreased with the increase in S/C, in agreement with the thermodynamic results.The conversion of LPG decreased with the increase in SV（6） The stability of the15%Ni/Mg_1.25Al-500catalyst was investigated. The resultshowed that the15%Ni/Mg_1.25Al-500catalyst had excellent catalytic stability during the reaction period of ca.20h. Compared with Ni/MgmAl catalysts calcined at hightemperature （800oC）, the present catalysts showed easier coke deposition and aslight decrease in catalytic activity and stability. However, it may be more promisingcandidate for LPG pre-reforming for the use of hydrogen fuel cells in transportationand small stationary applications due to lower reduction temperature.